1. Field of the Invention
This invention relates to rotary shaft seals, and particularly to seal assemblies adapted to be installed in cylindrical bores concentric with the axis of the rotary shaft.
2. Description of Prior Developments
Polytetrafluoroethylene has been used to form the sealing lip in rotary shaft seals because of its resistance to elevated operating temperatures, resistance to chemical attack, and resistance to wear. An example of a shaft seal assembly using polytetrafluoroethylene as a shaft seal lip element is shown in U.S. Pat. No. 3,462,333 to McCormick, et al.
In many cases the polytetrafluoroethylene seal element is mounted within an annular metallic seal case that has an outer annular surface adapted to fit into a cylindrical bore in a housing wall that serves as a mount for an anti-friction shaft bearing. It is necessary that the annular peripheral joint between the metal seal case and cylindrical bore be adequately sealed against liquid flow. In the past, this seal between the seal casing and bore has sometimes been formed by molding or coating an annular layer of rubber or elastomer onto the outer surface of the seal case as shown for example in U.S. Pat. No. 3,682,488 to Matsushima.
In many cases, heat is required to subsequently mold or bond the outer annular elastomeric seal to the metallic case. Such heat can have an adverse effect on the polytetrafluoroethylene shaft seal element, particularly as regards its dimensional stability, resilience and wear characteristics.
Because polytetrafluoroethylene must be heated to its gel temperature of about 621.degree. F. to bond the PTFE to the seal case, any layer of elastomer applied to the metal seal case must be applied after the polytetrafluoroethylene-to-metal bond has been completed. Otherwise, the elastomer will be degraded or rendered inoperative by the high temperatures required for polytetrafluoro-ethylene bonding. Thus, the elastomeric seal material must be applied to the seal case after the PTFE bonding operation.
Accordingly, a need exists for a shaft seal having a PTFE sealing element bonded to a metal seal casing and which includes an elastomeric sleeve bonded around the outer surface of the seal case without adversely affecting either the PTFE sealing element or the elastomeric sleeve.